There is an issue with 850s but I don't think that's it. The issue seems to be that different scenarios have generally very different upper temperature thresholds. As a general guide these are the T850 threasholds for a few different scenarios:
Windless secondary low cold sector: -1C
Ana Warm/Occluded front: -3C
Convective snow Inland: -6C
Fast moving Ana cold front: -6C
Convective snow on coast: -8C
Kata front: -10C
The issue is that people use, rightfully, a benchmark of -6C or -7C for snow showers which works but then assume that its transferable to frontal snow events. It isn't. The vast majority of frontal snow events allow substantially warmer uppers than convective snow. The main reasons for this is humidity and wind strength. Convective snow is often associated with strong winds and a moist environment which means evaporational cooling is virtually non existent especially if you reside near the coast. However for the case of a windless secondary low with heavy precipitation falling into dry air; evaporational cooling is very significant indeed. Evaporation is ridiculously efficient at removing heat from the atmosphere; indeed snow is possible with positive uppers if the precipitation lasts long enough in a windless, increasing humidity, environment.
Originally Posted by: Quantum